This invention relates to variable area jet nozzles such as are used, for example, as the exhaust outlets of gas turbine engines. More particularly, this invention relates to apparatus for sealing between areamodulating nozzle flaps to minimize flow leakage.
Gas turbine engines employ exhaust nozzles to direct the hot gases of combustion rearward into the atmosphere at a velocity and a density necessary to produce the required thrust. Essentially, the energy of the gases in the form of heat and pressure is converted into thrust. The area of the exhaust nozzle is extremely important since it is a determining factor of the efficiency with which thrust is produced. The choice of nozzle area is determined, in part, by turbine inlet temperature, mass airflow rate, and the velocity and pressure of the exhaust gases.
For engines with narrow operating ranges, the nozzle size is optimized during manufacturing and remains constant throughout its operation. Little is to be gained in performance of such an engine by use of variable area nozzles, and any possible benefits are generally outweighed by problems of weight, cost and complexity. On the other hand, it is well known that in advanced engines with broad operating ranges, noise thrust and fuel economy benefits may be achieved by use of variable area nozzles.
Traditionally, variable area nozzles have been adapted to engines having some sort of thrust augmentation, such as afterburner or preturbine injection. By increasing the nozzle area, the potentially higher temperatures associated with augmentation can be maintained at tolerable levels. Typically, a variable area nozzle is opened during low altitude take-off and, at the appropriate altitude after take-off, the nozzle is closed in order to achieve the necessary cruise thrust.
For supersonic flight a convergent-divergent nozzle is required, and such a nozzle must be of the variable area variety if the operating envelope of the engine is very large. Such a nozzle has a convergent portion designed to keep the exhaust gases subsonic until they reach the throat (point of minimum area) at which time they reach sonic velocity. The divergent portion then allows controlled expansion of the gases to supersonic velocities. Most modern variable area exhaust nozzles of the convergent-divergent variety make use of a plurality of pivotable flaps and any required area modulation is accomplished by actuation of these flaps into and out of the exhaust stream. Convergent flaps are used upstream of the throat and divergent flaps are employed downstream thereof.
Two basic means have been employed for connecting the convergent and divergent flaps at the throat. In one type, the flaps are connected at the throat by means of a hinge, with the downstream end of the divergent flap free to move in an axial, as well as radial, direction. However, some of the advanced two-dimensional nozzles, such as that taught and claimed in copending U.S. patent application Ser. No. 572,340 -Nash et al now U.S. Pat. No. 4,000,610 which is assigned the same assignee as the present invention and the disclosure of which is incorporated herein by reference, because of their construction, require that the downstream end of the divergent flap be pivoted at a fixed point. With both the forward and aft flap pivots fixed against axial translation, it becomes necessary to provide for sliding motion at the throat to accomplish throat area modulation. This sliding motion at the throat has been a continual problem in nozzle construction since there is inherent interference between the structural ribs on one flap and the surface of the other.
When nozzle flap motion has been small, this interference has been eliminated by undercutting the structural ribs to clear the flap skin or by employing slots in the skin to clear the ribs, or by a combination of both. However, where large flap motions are required, as in the case of the aforementioned advanced two-dimensional nozzles, or in axisymmetrical nozzles requiring a large area variation range, these are not practical solutions. Large flap motion would require structurally impractical undercutting of the flap structure and/or extensive slots in the flap skin with attendant leakage and performance problems. It is particularly important in such advanced nozzles that propulsion gas leakage between the flaps at the throat be minimized since such leakage leads to nozzle inefficiency. Furthermore, nozzle coolant gas leakage into the flow path may result from gaps between the flaps when the nozzle is of the air-cooled variety, thus increasing coolant flow requirements and penalizing engine cycle performance.